FIG. 1 shows a thin film CMOS device including a source 101, drain 102, and MOS gate region 103. The SOI layer 104 forms an MOS junction 105 with an buried oxide layer 106. Below buried oxide layer 106 is a substrate layer 107, which would typically be several hundred microns thick. On the scale shown in FIG. 1, the substrate layer 107 is too thick to depict and thus is not fully shown.
In source follower mode, sometimes called source high mode, the source 101 is biased with a voltage that is typically higher than the voltage at which substrate 107 is kept. This voltage difference could be over two hundred volts in typical applications. In thin film devices where the SOI layer 104 may be only slightly more than a micron thick, this voltage difference may be sufficient to induce an unwanted depletion region at or near the MOS junction 105. As shown therefore in FIG. 1, a parasitic path 110 between source 101 and drain 102 exists at the MOS junction. This region creates a parasitic MOS channel, allowing leakage current to be conveyed between the source 101 and drain 101 when the real MOS gate region 103 is intended to be turned off. The device thus undesirably acts as if a second gate region existed, wherein the second gate is in the on state even when the actual gate region 103 is in the off state.
To date, there exists no known solution for stopping this leakage current when thin film SOI devices are utilized in the source follower configuration. Prior solutions all involve use of a much thicker SOI layer 104, rather than thin film devices. These prior devices have such a thick SOI layer that the depletion region resulting in the parasitic MOS channel 110 does not occur. However, in thin film applications the region 110 acts as a second current path in addition to the normal gate region.
There exists a need in the art for a technique of eliminating this parasitic channel 110 in source follower configurations with thin film SOI devices.
In complimentary arrangements, (i.e. using NMOS devices in which the source is biased much lower than the substrate) a similar problem may exist.